Caution: Underground service-entrance cable (USE) is not permitted for interior wiring because it does not have a flame-retardant insulation. It would only be permitted in interior wiring when dual listed as wire type in accordance with Table 310.104, such as RHW.

• If subject to physical damage unless protected in accordance with 230.50(A).
• Underground with or without a raceway.
• Underground Service-Entrance Cable. USE cable isn’t permitted:
• For interior wiring.
• Above ground, except where protected against physical damage in accordance with 300.5(D).

Q2. What are the rules for minimum clearance above ground for overhead conductors?

A2. Overhead conductor spans must maintain vertical clearances as follows [225.18]:

• 10 ft above finished grade, sidewalks, platforms, or projections from which they might be accessible to pedestrians for 120V, 120/208V, 120/240V, or 240V circuits.
• 12 ft above residential property and driveways, and those commercial areas not subject to truck traffic for 120V, 120/208V, 120/240V, 240V, 277V, 277/480V, or 480V circuits.
• 18 ft over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles (such as those used for cultivation, grazing, forestry, and orchards).
• 24½ ft over track rails of railroads.

Author’s Comment: Overhead conductors located above pools, outdoor spas, outdoor hot tubs, diving structures, observation stands, towers, or platforms must be installed in accordance with the clearance requirements in 680.8.

Q3. What are the requirements for unused openings in enclosures?

A3. Unused openings, other than those intended for the operation of equipment or for mounting purposes, or those that are part of the design for listed products, must be closed by fittings that provide protection substantially equivalent to the wall of the equipment [110.12(A)

Note: Accepted industry practices are described in ANSI/NECA 1, Standard Practices for Good Workmanship in Electrical Contracting.

Author’s Comment: The National Electrical Contractors Association (NECA) created a series of National Electrical Installation Standards (NEIS)® that established the industry’s first quality guidelines for electrical installations. These standards define a benchmark or baseline of quality and workmanship for installing electrical products and systems. They explain what installing electrical products and systems in a “neat and workmanlike manner” means. For more information about these standards, visit www.neca-neis.org/.

Q4. What are the Code rules for identifying circuit conductors?

A4. The neutral conductor of a branch circuit must be identified in accordance with 200.6 [210.5(A)].

Equipment grounding conductors can be bare, covered, or insulated. Insulated equipment grounding conductors size 6 AWG and smaller must have a continuous outer finish either green or green with one or more yellow stripes, in conformance with 250.119 [210.5(B)].

On equipment grounding conductors 4 AWG and larger, insulation can be permanently reidentified with green marking at the time of installation at every point where the conductor is accessible [250.119(A)].

Ungrounded conductors must be identified as follows [210.5(C)]:

• If the premises wiring system contains branch circuits supplied from more than one voltage system, each ungrounded conductor must be identified by phase and system at all termination, connection, and splice points.
• Identification can be by color coding, marking tape, tagging, or other means approved by the authority having jurisdiction.
• The method of identification must be documented in a manner that’s readily available or permanently posted at each branch-circuit panelboard.

Author’s Comments:
•  When a premises has more than one voltage system supplying branch circuits, the ungrounded conductors must be identified by phase and system. This can be done by permanently posting an identification legend that describes the method used, such as color-coded marking tape or color-coded insulation.
•  Conductors with insulation that’s green or green with one or more yellow stripes can’t be used for an ungrounded or neutral conductor [250.119].
•  Although the NEC doesn’t require a specific color code for ungrounded conductors, electricians often use the following color system for power and lighting conductor identification:
   –  120/240V, single-phase—black, red, and white
   –  120/208V, three-phase—black, red, blue, and white
   –  120/240V, three-phase—black, orange, blue, and white
   –  277/480V, three-phase—brown, orange, yellow, and gray; or, brown, purple, yellow, and gray

Q5. What is the Code requirement for the use of slash versus straight voltage rated breakers?

A5. A circuit breaker with a straight voltage rating, such as 240V or 480V, is permitted on a circuit where the nominal voltage between any two conductors (line-to-neutral or line-to-line) doesn’t exceed the circuit breaker’s voltage rating [240.85].

A circuit breaker with a slash rating, such as 120/240V or 277/480V, is permitted on a solidly grounded system where the nominal voltage of any one conductor to ground doesn’t exceed the lower of the two values, and the nominal voltage between any two conductors doesn’t exceed the higher value.

Caution: A 120/240V slash circuit breaker must not be used on the high leg of a solidly grounded 4-wire, three-phase, 120/240V delta-connected system, because the line-to-ground voltage of the high leg is 208V, which exceeds the 120V line-to-ground voltage rating of the breaker.

Note: When installing circuit breakers on corner-grounded delta systems, consideration needs to be given to the circuit breakers’ individual pole-interrupting capability.

Q6. What are the grounding electrode requirements for a separately derived system, such as a transformer? [250.30(A)(4)]

A6. First of all, what is a separately derived system?
•  According to Article 100, a separately derived system is a wiring system whose power is derived from a source where there’s no direct electrical connection to the supply conductors of another system.
•  Transformers are considered separately derived when the primary conductors have no direct electrical connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.
•  A generator having transfer equipment that switches the neutral conductor, or one that has no neutral conductor at all, is a separately derived system and must be grounded and bonded in accordance with 250.30(A).

Note 2: For nonseparately derived systems, see 445.13 for the minimum size neutral conductors necessary to carry fault current.

Separately derived systems must be grounded and bonded in accordance with (A)(1) through (A)(8) [250.30(A)].

A neutral-to-case connection must not be made on the load side of the system bonding jumper, except as permitted by 250.142(B).

CAUTION: Dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment as well as metal piping and structural steel, in violation of 250.6(A), if more than one system bonding jumper is installed, or if it’s not located where the grounding electrode conductor terminates to the neutral conductor.

(1) An unspliced system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the neutral terminal of the separately derived system; either at the separately derived system or the system disconnecting means, but not at both locations [250.30(A)(5)].

Author’s Comment: A system bonding jumper is the connection between the neutral conductor and supply side bonding jumper or equipment grounding conductor or both at a separately derived system [Article 100].

• Where the system bonding jumper is installed at the source of the separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the source (transformer case).
• Where the system bonding jumper is installed at the first disconnecting means of a separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal disconnecting means enclosure.

Author’s Comment: A system bonding jumper is a conductor, screw, or strap that bonds the metal parts of a separately derived system to the system neutral point [Article 100 Bonding Jumper, System], and it’s sized to Table 250.66 in accordance with 250.28(D).

DANGER: During a ground fault, metal parts of electrical equipment, as well as metal piping and structural steel, will become and remain energized providing the potential for electric shock and fire if the system bonding jumper isn’t installed.

(2) Supply-Side Bonding Jumper. If the separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper must be run to the derived system disconnecting means. The supply-side bonding jumper can be a nonflexible metal raceway, a wire, or a bus.

(a)     If the supply-side bonding jumper is of the wire type, it must be sized in accordance with Table 250.66, based on the area of the largest ungrounded derived system conductor in the raceway or cable.

Question: What size supply-side bonding jumper is required for flexible metal conduit containing 300 kcmil secondary conductors?
(a) 3 AWG                    (b) 2 AWG          (c) 1 AWG      (d) 1/0 AWG
Answer: (b) 2 AWG [Table 250.66]

(b)     If the supply-side bonding jumper is a bus, it must have a cross-sectional area no smaller than required by Table 250.66.

(3) If the system bonding jumper is installed at the disconnecting means instead of at the source, the following requirements apply when sizing the system neutral conductor:

• Sizing for Single Raceway. Because the neutral conductor of a derived system serves as the effective ground-fault current path for ground-fault current, it must be routed with the ungrounded conductors of the derived system and be sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor of the derived system.
• Parallel Conductors in Two or More Raceways. If the conductors from the derived system are installed in parallel in two or more raceways, the neutral conductor of the derived system in each raceway or cable must be sized not smaller than specified in Table 250.66, based on the area of the largest ungrounded conductor of the derived system in the raceway or cable. In no case is the neutral conductor of the derived system permitted to be smaller than 1/0 AWG [310.10(H)].

Author’s Comment: If the system bonding jumper is installed at the disconnecting means instead of at the source, an equipment bonding conductor must connect the metal parts of the separately derived system to the neutral conductor at the disconnecting means in accordance with 250.30(A)(2).

(4) Grounding Electrode. The grounding electrode must be as near as practicable, and preferably in the same area where the system bonding jumper is installed and be one of the following:

(1)     Metal water pipe electrode, within 5 ft of the entry to the building [250.52(A)(1)].

(2)     Metal building frame electrode [250.52(A)(2)].

Ex 1: If the electrodes specified in 250.30(A)(4) aren’t available, one of the following electrodes can be used:

•    A concrete-encased electrode encased by not less than 2 in. of concrete, located horizontally near the bottom or vertically, and within that portion of concrete foundation or footing that’s in direct contact with the earth [250.52(A)(3)].
•    A ground ring electrode encircling the building/structure, buried not less than 30 in. below grade, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4) and 250.53(F)].
•    A ground rod electrode having not less than 8 ft of contact with the soil meeting the requirements of 250.52(A)(5) and 250.53(G)].
•    Other metal underground systems, piping systems, or underground tanks [250.52(A)(8)].

Note 1: Interior metal water piping in the area served by separately derived systems must be bonded to the separately derived system in accordance with 250.104(D).

(5) Grounding Electrode Conductor, Single Separately Derived System. The grounding electrode conductor must be sized in accordance with 250.66, based on the area of the largest ungrounded conductor of the derived system. A grounding electrode conductor must connect the neutral terminal of a separately derived system to a grounding electrode of a type identified in 250.30(A)(4) at the same point on the separately derived system where the system bonding jumper is connected.

Author’s Comments:
•  System grounding also helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components.
•  To prevent objectionable neutral current from flowing [250.6] onto metal parts, the grounding electrode conductor must originate at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)].

Ex 1: If the system bonding jumper is a wire or busbar, the grounding electrode conductor is permitted to terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1).

Ex 3: Separately derived systems rated 1 kVA or less aren’t required to be grounded (connected to the earth).

(6) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where there are multiple separately derived systems, a grounding electrode conductor tap from each separately derived system to a common grounding electrode conductor is permitted. This connection is to be made at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)].

Ex 1: If the system bonding jumper is a wire or busbar, the grounding electrode conductor tap can terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1).

Ex 2: Separately derived systems rated 1 kVA or less aren’t required to be grounded (connected to the earth).

Q7. What types of conductors are permitted within raceways located in wet locations?

A7. Insulated conductors and cables installed in raceways in aboveground wet locations must be listed for use in wet locations according to 310.10(C) [300.9].

Q8. When are receptacles required to be tamper resistant?

A8. All nonlocking type 15A and 20A, 125V receptacles in the following areas of a dwelling unit [210.52] must be listed as tamper resistant [406.12].
•    Wall Space—210.52(A)
•    Small-Appliance Circuit—210.52(B)
•    Countertop Space—210.52(C)
•    Bathroom Area—210.52(D)
•    Outdoors—210.52(E)
•    Laundry Area—210.52(F)
•    Garage and Outbuildings—210.52(G)
•    Hallways—210.52(H)

Receptacles in the following locations aren’t required to be tamper-resistant [406.12 Ex.]:
(1)     Receptacles located more than 5½ ft above the floor.
(2)     Receptacles that are part of a luminaire or appliance.
(3)     A receptacle located within dedicated space for an appliance that in normal use isn’t easily moved from one place to another.
(4)     Nongrounding receptacles used for replacements as permitted in 406.4(D)(2)(a).

Q9. What are the Code rules for the overcurrent protection of panelboards?

A9. Each panelboard must be provided with overcurrent protection located within, or at any point on the supply side of, the panelboard. The overcurrent device must have a rating not greater than that of the panelboard, and it can be located within or on the supply side of the panelboard [408.36].

Ex 1: Individual overcurrent protection isn’t required for panelboards used as service equipment in accordance with 230.71.

When a panelboard is supplied from a transformer, as permitted in 240.21(C), the overcurrent protection for the panelboard must be on the secondary side of the transformer. The required overcurrent protection can be in a separate enclosure ahead of the panelboard, or it can be in the panelboard [408.36(B)].

Plug-in circuit breakers that are back-fed from field-installed conductors must be secured in place by an additional fastener that requires other than a pull to release the breaker from the panelboard [408.36(D)].

Author’s Comments:
•  The purpose of the breaker fastener is to prevent the circuit breaker from being accidentally removed from the panelboard while energized, thereby exposing someone to dangerous voltage.
•  For photovoltaic systems, conductors from the PV ac inverter is permitted to backfed dedicated circuit breakers that aren’t marked “Line” and “Load” [705.12(D)(5)].

Caution: Circuit breakers marked “Line” and “Load” must be installed in accordance with listing or labeling instructions [110.3(B)]; therefore, these types of devices must not be back-fed.

Q10.  What are the GFCI protection requirements of space-heating cables?

A10. GFCI protection is required for electric space-heating cables that are embedded in concrete or poured masonry floors of bathrooms, kitchens, and hydromassage bathtub locations [424.44(G)].

Author’s Comment: See 680.28(C)(3) for restrictions on the installation of radiant-heating cables for spas and hot tubs installed outdoors.

Q11. What is the NEC rule regarding objectionable current?

A11. To prevent a fire, electric shock, or improper operation of circuit overcurrent devices or electronic equipment, electrical systems and equipment must be installed in a manner that prevents objectionable neutral current from flowing on metal parts [250.6].

Temporary currents from abnormal conditions, such as ground faults, aren’t to be classified as objectionable current [250.6(C)].

Currents that introduce noise or data errors in electronic equipment are not considered objectionable currents for the purposes of this section. Circuits that supply electronic equipment must be connected to an equipment grounding conductor [250.6(D)].

OBJECTIONABLE CURRENT

Objectionable neutral current occurs because of improper neutral-to-case connections or wiring errors that violate 250.142(B).

Improper Neutral-to-Case Connection [250.142]

Panelboards. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a panelboard that’s not used as service equipment.

Separately Derived Systems. Objectionable neutral current will flow on conductive metal parts and conductors if the neutral conductor is connected to the circuit equipment grounding conductor on the load side of the system bonding jumper for a separately derived system.

Disconnects. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a disconnecting means that’s not part of the service equipment.

Wiring Errors. Objectionable neutral current will flow when the neutral conductor from one system is connected to a circuit of a different system.

Objectionable neutral current will flow on metal parts when the circuit equipment grounding conductor is used as a neutral conductor such as where:
•    A 230V time-clock motor is replaced with a 115V time-clock motor, and the circuit equipment grounding conductor is used for neutral return current.
•    A 115V water filter is wired to a 240V well-pump motor circuit, and the circuit equipment grounding conductor is used for neutral return current.
•    The circuit equipment grounding conductor is used for neutral return current.

DANGERS OF OBJECTIONABLE CURRENT

Objectionable neutral current on metal parts can cause electric shock, fires, and improper operation of electronic equipment and overcurrent devices such as GFPs, GFCIs, and AFCIs.

Shock Hazard. When objectionable neutral current flows on metal parts, electric shock and even death can occur from the elevated voltage on those metal parts.

Fire Hazard. When objectionable neutral current flows on metal parts, a fire can ignite adjacent combustible material. Heat is generated whenever current flows, particularly over high-resistance parts. In addition, arcing at loose connections is especially dangerous in areas containing easily ignitible and explosive gases, vapors, or dust.

Improper Operation of Electronic Equipment. Objectionable neutral current flowing on metal parts of electrical equipment and building parts can cause electromagnetic fields which negatively affect the performance of electronic devices, particularly medical equipment. For more information, visit www.MikeHolt.com, click on the “Technical Link,” and then on “Power Quality.”

When a system is properly grounded and bonded, the voltage of all metal parts to the earth and to each other will be zero.

When objectionable neutral current travels on metal parts because of the improper bonding of the neutral to metal parts in violation of the NEC, a difference of potential will exist between all metal parts. This situation can cause some electronic equipment to operate improperly.

Operation of Overcurrent Devices. When objectionable neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with ground-fault protection can occur because some neutral current flows on the circuit equipment grounding conductor instead of the neutral conductor.

Q12. When is equipment required to be ‘field’ marked with available fault current?

A12. Service equipment in other than dwelling units must be legibly field-marked with the maximum available fault current, including the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved [110.24(A)].

When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current [110.24(B)].

Ex: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

Q13. What is a Class I Hazardous (Classified) location?

A13. Locations are classified according to the properties of the flammable gases, flammable liquid-produced vapors, combustible liquid-produced vapors, combustible dusts, or fibers/flyings that may be present, and the likelihood that a flammable or combustible concentration will be present [500.5(A)].

Note: To reduce expensive equipment and expensive wiring methods, locate as much electrical equipment as possible in an unclassified location.

Each room, section, or area is considered individually in determining its classification. The same building/structure might contain both Class I, Division 1 and 2 locations, or Class II, Division 1 and 2 locations, or Class III, Division 1 and 2 locations.

Author’s Comment: See the definitions of “Building” and “Structure” in Article 100.

A Class I location is an area where flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors may be present in the air and in quantities sufficient to produce explosive or ignitible mixtures [500.5(B)].

An area where ignitible concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors may exist in the course of normal operations [500.5(A)(1)]:

• Continuously or periodically under normal operating conditions.
• Because of repair or maintenance operations or leakage.
• If faulty equipment releases ignitible concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors and the equipment becomes a source of ignition.

Note: Class I, Division 1 locations include:
•    Areas where volatile flammable liquids or liquefied flammable gases are transferred from one container to another, such as at gasoline storage and dispensing areas.
•    Interiors of spray booths and in the vicinity of spraying and painting operations where volatile flammable solvents are used to coat products with paint or plastics.
•    Locations containing open tanks or vats of volatile flammable liquids, or dip tanks for parts cleaning or other operations.
•    See 500.5(B)(3) Note 1 and 2 in the NEC for more information.

A Class I, Division 2 Location is an area where volatile flammable gases or vapors would become hazardous only in case of an accident or of some unusual operating condition, or under any of the following conditions [500.5(A)(2)]:

• If volatile flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are handled, processed, or used, but are normally confined within closed containers and the gases would only escape in the case of accidental rupture or breakdown, or in case of abnormal operation of equipment.
• If ignitible concentrations of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are normally prevented by positive mechanical ventilation, but might become hazardous through failure or abnormal operation of the ventilating equipment.
• Areas adjacent to a Class I, Division 1 location and to where flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors might occasionally be communicated unless prevented by adequate positive-pressure ventilation with effective safeguards against ventilation failure.

Note 1: The quantity of volatile flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors that might escape in case of accident, the adequacy of ventilating equipment, the total area involved, and the record of the industry with respect to explosions or fires are all factors that should be taken into consideration.

Q14. What is a Class II Hazardous (Classified) location?

A14. Class II locations are those where the presence of combustible dust may be suspended in the air with quantities sufficient to ignite or explode [500.5(C)].

A Class II, Division 1 location is an area where combustible dust may exist under any of the following conditions [500.5(C)(1)]:

• Nonconductive combustible dust is continuously or periodically suspended in the air in sufficient quantities to produce mixtures that will ignite or explode.
• If faulty equipment releases ignitible mixtures of dust and the equipment becomes a source of ignition.

An area where combustible dust would become hazardous under any of the following conditions [500.5(C)(2)]:

• If combustible dust, due to abnormal operations, may be present in the air in quantities sufficient to produce explosive or ignitible mixtures, or
• If combustible dust accumulation is normally insufficient to interfere with the normal operation of electrical equipment, but where malfunctioning of equipment may result in combustible dust being suspended in the air, or
• If combustible dust accumulations on, in, or near electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitible by abnormal operation or failure of electrical equipment.

Note 1: The quantity of combustible dust that may be present and the adequacy of dust removal systems should be considered when determining the area classification.

Q15. What is a Class III Hazardous (Classified) location?

A15. A Class III location is an area where easily ignitible fibers or materials producing combustible flyings are handled, manufactured, or used and aren’t likely to be suspended in the air in quantities sufficient to produce ignitible mixtures [500.5(D)].

A Class III, Division 1 Location is an area where ignitible fibers/flyings are manufactured, handled, or used, such as within textile mills or clothing manufacturing plants, as well as in facilities that create sawdust and flyings by pulverizing or cutting wood [500.5(D)(1)].

A Class III, Division 2 Location is an area where ignitible fibers/flyings are stored or handled other than in the manufacturing process [500.5(D)(2)].

Q16. What are the NEC requirements on the termination of the grounding electrode conductor to a grounding electrode?

A16. The mechanical elements used to terminate a grounding electrode conductor or bonding jumper to a grounding electrode must be accessible [250.68(A)].

Ex 1: The termination isn’t required to be accessible if the termination to the electrode is encased in concrete or buried in the earth.

Author’s Comment: If the grounding electrode attachment fitting is encased in concrete or buried in the earth, it must be listed for direct soil burial or concrete encasement [250.70].

Ex 2: Exothermic or irreversible compression connections, together with the mechanical means used to attach to fireproofed structural metal, aren’t required to be accessible.

A bonding jumper must be installed around insulated joints and equipment likely to be disconnected for repairs or replacement for an underground metal water piping system used as a grounding electrode. The bonding jumper must be of sufficient length to allow the removal of such equipment while retaining the integrity of the grounding path [250.68(B)].

For termination to metal water pipe and structural metal, grounding electrode conductors and grounding electrode bonding jumpers are permitted to terminate to [250.68(C)]:

• Interior metal water piping located not more than 5 ft from the point of entrance to the building/structure [250.68(C)(1)].

Ex: In industrial, institutional, and commercial buildings where conditions of maintenance and supervision ensure only qualified persons service the installation, the entire length of the metal water piping system can be used for grounding purposes, provided the entire length, other than short sections passing through walls, floors, or ceilings, is exposed.

• The metal frame of a building/structure that’s in direct contact with the earth for 10 ft or more [250.52(A)(2)] or connected to one or more of the following [250.68(C)(2)]:

               (a) Concrete-encased electrode [250.52(A)(3)] or ground ring [250.52(A)(4)],
               (b) Ground rod [250.52(A)(5)],
               (c) Other approved earth connection.

Q17.  In a major repair garage where gasoline or gaseous fuels are not dispensed, what are the classification rules?

A17. In major repair garages, if gasoline, or gaseous fuels, such as natural gas, hydrogen, or LPG, won’t be dispensed, the classification rules in (1), (2), and (3) apply [511.3(C)]:

(1) Floor Areas. Ventilation Provided. The floor area can be unclassified if there’s a minimum of four air changes per hour for each square foot of floor area.

• Ventilation Not Provided. The entire floor area is classified as Class I, Division 2 up to 18 in. above the floor.

(2) If vehicles fueled with natural gas or hydrogen are repaired or stored, the area within 18 in. of the ceiling is classified in accordance with (a) and (b).

(a) The ceiling area is unclassified if ventilation is provided from not more than 18 in. from the highest point in the ceiling to exhaust the ceiling area at a rate of at least 1 cfm/sq ft at all times that the building is occupied, or when vehicles using lighter-than-air gaseous fuels are parked below this area.

(b) Ventilation Not Provided. The ceiling area is classified as Class I, Division 2.

(3) Pit Areas in Lubrication or Service Room. The pit area is classified as provided in (a) or (b).

(a) Ventilation Provided. The pit area is classified as Class I, Division 2 if there’s a minimum of six air changes per hour.

(b) Ventilation Not Provided. The pit area is classified as Class I, Division 1 up to the floor level.

Q18. In a minor repair garage where flammable liquids won’t be dispensed or transferred, what are the classification rules?

A18. In minor repair garages, if flammable liquids won’t be dispensed or transferred, the classification of (D)(1), (D)(2), and (D)(3) apply [511.3(D)].

(1) Floor Areas. Floor areas are unclassified, except pit areas are classified as follows:

• The pit area can be unclassified if there’s a minimum of four air changes per hour.
• Ventilation Not Provided: The pit area is classified as Class I, Division 2 up to the floor level.

(2) Ceiling Areas. The ceiling area is unclassified if natural gas or hydrogen won’t be transferred.

(3) Pit Areas in Lubrication or Service Room. Pit areas must be classified according to (a) or (b).

• Ventilation Provided: The pit area is unclassified if there’s a minimum of 1 cfm/sq ft of the floor area at all times that the building is occupied or when vehicles are parked in or over this area.
• Ventilation Not Provided: The pit area is classified as Class I, Division 2 up to 18 in. above floor level and extending out 3 ft.

Areas adjacent to classified locations aren’t classified if mechanically ventilated at a rate of four or more air changes per hour, or when walls or partitions effectively cut off the adjacent area [511.3(E)(1)].

The storage, handling, or dispensing into motor vehicles of alcohol-based windshield washer fluid in areas used for the service and repair operations of the vehicles doesn’t cause such areas to be classified as hazardous (classified) locations [511.3(E)(2)].

Author’s Comment: Windshield washer fluid isn’t flammable.

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